Field of the Invention
The present invention relates to a cap assembly and a battery including the same and, more particularly, to a cap assembly for a cylindrical secondary battery, in which the support unit of a current interrupt device (CID) is seated in the inclined portion or the terraced portion of a gasket and thus the battery is dually sealed, thereby preventing the leakage of an electrolyte, and a cylindrical secondary battery including the cylindrical secondary battery.
Discussion of the Related Art
A secondary battery is classified into a cylindrical battery and an angular battery in which an electrode assembly is embedded in a cylindrical or angular metal can and a pouch type battery in which an electrode assembly is embedded in the pouch type casing of an aluminum laminate sheet, depending on the shape of the battery casing.
Further, the electrode assembly built in the battery casing is an electric generation device is configured to have a stack structure of a positive electrode, a separation film, and a negative electrode and can be charged and discharged. The electrode assembly is classified into a jelly-roll type structure in which the positive electrode and the negative electrode of a long sheet type, coated with an active material, are wound with the separation film interposed therebetween and a stack type structure in which the plurality of positive electrodes and the plurality of negative electrodes, having a specific size, are sequentially stacked with the separation film interposed therebetween. From among them, the electrode assembly of the jelly-roll type structure is most widely used because it is advantageous in terms of easy manufacture and a high energy density per weight. The electrode assembly of the jelly-roll type structure is chiefly used for the cylindrical battery.
However, the electrode assembly of the jelly-roll type structure is likely to be deformed while experiencing repetitive expansion and contraction when the battery is charged and discharged. In this process, stress is concentrated on a central portion of the electrode assembly, and so the electrode penetrates the separation film and then brings into contact with a metal center pin, thereby causing an internal short. An organic solvent can be decomposed because of heat generated by the internal short and generate gas, and the battery can be exploded because of a rise in the gas pressure within the battery. Such a rise in the gas pressure within the battery can occur even when an internal short is generated by an external impact.
In order to solve the safety problem of the battery, safety elements, such as a safety vent for exhausting a high-pressure gas, a positive temperature coefficient (hereinafter referred to as a ‘PTC’) element for interrupting current at high temperature, and a current interrupt device (hereinafter referred to as a ‘CID’) for interrupting current when an internal pressure within the battery rises, and a top cap forming a projection type terminal for protecting the elements are fixed by a gasket within the cap assembly of the cylindrical battery.
The cap assembly is configured to prevent an electrolyte within the battery from fundamentally leaking externally in such a manner that the gasket surrounds the outer circumference, including the safety vent, the PTC element, the CID, and the top cap. Accordingly, if the electrolyte is not leaked through an interface of the safety vent placed on the innermost side of the battery and the gasket configured to surround the outer circumference of the safety vent, the electrolyte is not leaked through interfaces between metal materials, such as the interface of the safety vent and the PTC element and the interface of the PTC element and the top cap.
However, some of the electrolyte is substantially leaked through the interface of the gasket and the safety vent in a process of charging and discharging the battery, because of dropping and an external impact, and so on. There is a problem in that the electrolyte can be easily leaked externally through the interfaces between the metal materials. That is, the interface between the metal materials is relatively low in the adhesion strength. Accordingly, the electrolyte once introduced into the interface between the metal materials can easily leak externally as compared with the interface between the gasket and its pertinent elements.
Accordingly, there is a great need for a technique capable of reducing a phenomenon in which the electrolyte is leaked from the cap assembly.
In line with the necessity, Japanese Unexamined Patent Application Publication No. 2006-286561, Japanese Unexamined Patent Application Publication No. 2005-100927, Japanese Unexamined Patent Application Publication No. 2002-373711, etc. disclose a cap assembly having a gasket provided under a top cap. However, the cap assembly disclosed in the patent application publications is difficult to manufacture because the gasket to surround and seal the outer circumference of safety devices has a complicated shape and does not fundamentally solve the above problems because an electrolyte is leaked from the interface between the metal materials (the safety vent, the PTC element, and the top cap).
FIG. 1 is a cross-sectional view showing the upper structure of a conventional cylindrical secondary battery.
Referring to FIG. 1, the battery 100 is fabricated by inserting an electrode assembly 300 (i.e., an electric generation device) into a can 200, injecting an electrolyte into the can 200, and mounting a cap assembly 400 on the upper opening of the can 200.
The cap assembly 400 includes a top cap 410, a PTC element 420 for interrupting overcurrent, and a safety vent 430 for lowering an internal pressure. The top cap 410, the PTC element 420, and the safety vent 430 are closely adhered and disposed within a gasket 500 configured to maintain airtightness and mounted on the upper beading unit 210 of the can 200.
The top cap 410 has a central portion projected upward and functions as a positive electrode terminal through a contact with an external circuit. A plurality of penetration holes (not shown) for exhausting gas is perforated in the top cap 410. The safety vent 430 has its bottom connected to the positive electrode of the electrode assembly 300 via a current interruption safety device 440 and a positive electrode lead 310.
The safety vent 430 is made of a thin conductive sheet material. A downward indentation portion 432 is formed in a central portion of the safety vent 430, and two notches with different depths are formed at the upper curved portion and the lower curved portion of the downward indentation portion 432.
The current interruption safety device 440 is disposed under the safety vent 430 made of the conductive sheet material and it functions to interrupt current when pressure within the battery is higher than a critical value. The current interruption safety device 440 preferably is made of the same material as the safety vent 430. An auxiliary gasket 510 is made of polypropylene (PP)-based materials so that it can prevent the current interruption safety device 440 and the safety vent 430 from becoming electrified.
For example, when a temperature of the battery 100 rises because of an internal short, overcharge, etc. caused by various causes, the amount of electrified current is greatly reduced by an increase in the resistance of the PTC element 420. If the electrolyte is decomposed by a continued rise in the temperature, thus generating gas, and resultantly an internal pressure rises, the downward indentation portion 432 of the safety vent 430 is raised up and the current interruption safety device 440 is partially ruptured to interrupt current, thereby guaranteeing safety. If the pressure continues to rise, the notches 436 of the safety vent 430 are ruptured and so a high-pressure gas is exhausted outside the battery 100 in order to guarantee safety.